High-excitation SiO maser emission in VY Canis Majoris - Detection of the V = 4 J = 5-4 transition

We report the detection of the v = 4 J = 5-4 transition of SiO in VY CMa, together with 13 maser lines of the same molecule corresponding to the rotational lines J = 1-0 to J = 6-5 of the vibrational levels 1-3. This is the first time that maser emission arising from a level of about 7000 K has been observed. The line profile from this transition has two different peaks at extreme velocities, in contrast to the strong v = 1 J = 2-1 maser, which has an intense emission peak at 22 km/s, i.e., closer to the velocity of the star. The density necessary to produce these high-excitation masers is about 10 exp 10 cu cm, and the SiO column densities have to be higher than or about 10 exp 20/sq cm. The maser chains inside a given vibrational level predicted by standard large velocity gradient models (radiative or collisional pumping) are not observed for v = 3 and v = 4, indicating that nonlocal effects and infrared line overlaps could play an important role in the pumping of these high-excitation masers.